There has been proposed a solid state image pickup apparatus comprising, as main components, a solid state image pickup device having an image pickup surface composed of a prescribed pattern, such as a matrix for example, of two-dimensionally arrayed image pickup picture units each comprising a photoelectric transducer and a switching element and additional switching elements disposed outside of the image pickup picture units for successively delivering, in a predetermined manner, signals based on signal charge produced by the image pickup surface, and a scanning circuit for selectively energizing the switching elements in the solid state image pickup device to deliver an image pickup signal output based on signal charge produced by the photoelectric transducers in the image pickup picture units. In the solid state image pickup device employed in such a solid state image pickup apparatus, each switching element is composed of an insulated-gate field-effect transistor (hereinafter referred to as a "MOS FET"), and the photoelectric transducers are composed of a plurality of photodetecting diodes provided respectively for the switching elements or a thin photoelectric conversion layer disposed over the two-dimensional array of the switching elements.
FIG. 1 shows an equivalent circuit of the solid state image pickup device that has heretofore been proposed, and a scanning circuit and an output circuit which are required for putting the solid state image pickup device into operation. The solid state image pickup device, generally designated at 1, is composed of enhancement-mode MOS FETs S.sub.11 to S.sub.mn serving as switching elements, respectively, and arrayed as a matrix in horizontal rows (in the direction of an arrow H) and in vertical rows (in the direction of an arrow V), and photoelectric transducers D.sub.11 to D.sub.mn each connected to one end, for example, a source of the corresponding one of the MOS FETs S.sub.11 to S.sub.mn. These MOS FETs S.sub.11 to S.sub.mn and photoelectric transducers D.sub.11 to D.sub.mn form an image pickup surface. Each combination of one of the MOS FETs S.sub.11 to S.sub.mn and one of the photoelectric transducers D.sub.11 to D.sub.mn forms one of image pickup picture units E.sub.11 to E.sub.mn.
The gates of the MOS FETs S.sub.11 to S.sub.mn which constitute the image pickup picture units E.sub.11 to E.sub.mn are connected in common in each horizontal row, and the groups of the gates connected in common are coupled respectively to m control terminals v.sub.1 to v.sub.m which are supplied with vertical scanning signals from a vertical scanning circuit 2. The drains of the MOS FETs S.sub.11 to S.sub.mn are connected in common in each vertical row, and groups of the drains connected in common are coupled to the sources, respectively, of enhancement-mode MOS FETs T.sub.1 to T.sub.n serving as switching elements. The enhancement-mode MOS FETs T.sub.1 to T.sub.n have their gates connected respectively to n control terminals h.sub.1 to h.sub.m which are supplied with horizontal scanning signals from a horizontal scanning circuit 3. The drains of the MOS FETs T.sub.1 to T.sub.n are connected in common through an output resistor 4 to a power supply 5 which supplies an operation voltage V.sub.V. An output terminal 6 is led from a junction between the drains of the MOS FETs T.sub.1 to T.sub.n connected in common and the output resistor 4.
The vertical scanning circuit 2 includes a shift register, for example, for issuing vertical scanning signals through the m control terminals v.sub.1 to v.sub.m to the gates of the MOS FETs S.sub.11 to S.sub.mn to successively energize the rows of the MOS FETs S.sub.11 to S.sub.mn. The horizontal scanning circuit 3 includes a shift register, for example, for issuing horizontal scanning signals having a frequency sufficiently higher than that of the vertical scanning signals from the vertical scanning circuit 2 through the n control terminals h.sub.1 to h.sub.n to the gates of the MOS FETs T.sub.1 to T.sub.n to successively energize the MOS FETs T.sub.1 to T.sub.n.
The ends of the photoelectric transducers D.sub.11 to D.sub.mn remote from the sources of the MOS FETs S.sub.11 to S.sub.mn are connected in common to an output terminal 7 to which a predetermined voltage V.sub.T is applied. In the case where the photoelectric transducers D.sub.11 to D.sub.mn are composed of a plurality of photodiodes, for example, a ground potential is impressed on the terminal 7, and in the case where the photoelectric transducers D.sub.11 to D.sub.mn are formed with a thin photoelectric conversion layer, a potential corresponding to a predetermined voltage (target voltage) V.sub.T is applied to the terminal 7.
Any desired one E of the image pickup picture units E.sub.11 to E.sub.mn of the solid state image pickup device 1 thus constructed and a corresponding one T of the MOS FETs T.sub.1 to T.sub.n are connected as shown in FIG. 2. In this example, the photoelectric transducer is composed of a photodiode, and the image pickup picture unit E has a photoelectric transducer D and an enhancement-mode MOS FET S. The photoelectric transducer D has one terminal connected to the source of the MOS FET S. The drain of the MOS FET S is connected to the source of an enhancement-mode MOS FET T having a drain connected to the output terminal 6 and to the power supply 5 through the output resistor 4. The other terminal of the photoelectric transducer D is grounded. The gate of the MOS FET S is connected to one v of the control terminals v.sub.1 to v.sub.m, while the gate of the MOS FET T is connected to one h of the control terminals h.sub.1 to to h.sub.n.
When light from an object falls on the image pickup picture units E.sub.11 to E.sub.mn on the image pickup surface of the solid state image pickup device 1 thus constructed, the photoelectric transducers D.sub.11 to D.sub.mn convert light energy into electric charge (electrons) which is dependent on guantities of incident light falling respectively on the image pickup picture units E.sub.11 to E.sub.mn. The electric charge is then stored in the respective sources of the MOS FETs S.sub.11 to S.sub.mn as signal charge. A signal based on the stored signal charge is delivered to the output terminal 6 to produce an image pickup signal output when the horizontal rows of the MOS FETs S.sub.11 to S.sub.mn are selectively energized by the vertical scanning signals issued from the vertical scanning circuit 2 and the MOS FETs T.sub.1 to T.sub.n are selectively energized by the horizontal scanning signals issued from the horizontal scanning circuit 3.
For such vertical and horizontal scanning operation, the vertical scanning circuit 2 supplies to the m control terminals v.sub.1 to v.sub.m with vertical scanning signals .PHI..sub.v1 to .PHI..sub.vm, respectively, as shown in FIG. 3A, and the horizontal scanning circuit 3 supplies the n control terminals h.sub.1 to h.sub.n with horizontal scanning signals .PHI..sub.h1 to .PHI..sub.hn, respectively, as shown in FIG. 3B. More specifically, the vertical scanning signals .PHI..sub.vl to .PHI..sub.vm are composed of m pulses .phi.v1 to .phi..sub.vm which have a high level during a period t.sub.h corresponding to one horizontal period of a video signal and are generated successively in one vertical period. The horizontal scanning signals .PHI..sub.h1 to .phi..sub.hn are composed of n pulses .phi..sub.h1 to .phi..sub.hn which have a high level in a short period and are generated successively in the period of each of the pulses .phi..sub.v1 to .phi..sub.vm of the vertical scanning signals .PHI..sub.v1 to .PHI..sub.vm. The MOS FETs S.sub.11 to S.sub.mn and T.sub.1 to T.sub.n are energized when their gates are supplied with the pulses .phi..sub.v1 to .phi..sub.vm of the vertical scanning signals .phi..sub.v1 to .phi..sub.vm and the pulses .phi..sub.h1 to .phi..sub.hn of the horizontal scanning signals .PHI..sub.h1 to .PHI..sub.hn.
The pulse .phi..sub.v1 of the vertical scanning signal .PHI..sub.v1 delivered from the vertical scanning circuit 2 is first applied to the gates of the MOS FETs S.sub.11 to S.sub.1n which constitute the first row to energize the MOS FETs S.sub.11 to S.sub.1n, whereupon a signal based on the signal charge stored in their sources is transmitted to the sources of the MOS FETs T.sub.1 to T.sub.n. During the period of the pulse .phi..sub.v1, the pulses .phi..sub.h1 to .phi..sub.hn of the horizontal scanning signals .PHI..sub.h1 to .PHI..sub.hn from the horizontal scanning circuit 3 are successively applied to the gates of the MOS FETs T.sub.1 to T.sub.n, respectively, to energize the MOS FETs T.sub.1 to T.sub.n, successively. A signal current based on the signal that has been transmitted to the sources of the MOS FETs T.sub.1 to T.sub.n is now allowed to flow through the output resistor 4. As a consequence, an image pickup signal output produced by the image pickup picture units E.sub.11 to E.sub.1n corresponding respectively to the MOS FETs S.sub.11 to S.sub.1n becomes continuously available from the output terminal 6.
Then, the pulse .phi..sub.v2 of the vertical scanning signal .PHI..sub.v2 is supplied to the gates of the MOS FETs S.sub.21 to S.sub.2n which constitute the next horizontal row to energize these MOS FETs S.sub.21 to S.sub.2n, and the MOS FETs T.sub.1 to T.sub.n are successively rendered conductive by the pulses .phi..sub.h1 to .phi..sub.hn of the horizontal scanning signals .PHI..sub.h1 to .PHI..sub.hn. Therefore, an image pickup signal output produced by the image pickup picture units E.sub.21 to E.sub.2n corresponding respectively to the MOS FET S.sub.21 to S.sub.2n becomes continuously available from the output terminal 6. Likewise, image pickup signal outputs produced by the successive image pickup picture units up to the image pickup picture units E.sub.m1 to E.sub.mn corresponding respectively to the MOS FETs S.sub.m1 to S.sub.mn become sequentially available from the output terminal 6 in the respective periods each equivalent to one vertical period. The foregoing cycle of scanning operation will be repeated.
The derivation of the image pickup output signal from any desired one image pickup picture unit E on the image pickup surface of the foregoing prior solid state image pickup device 1 will be considered with reference to FIG. 2. First, it is supposed here that the MOS FET S constituting the image pickup picture unit E and the MOS FET T corresponding thereto are have been turned on to deliver an image pickup signal output from the image pickup picture unit E, and thereafter the MOS FET S and the MOS FET T are turned off again. At this time, a junction capacitance C.sub.j, shown by a broken line in FIG. 2, across a P-N junction which forms the photoelectric transducer (photodiode) D is connected to the source of the MOS FET S and a stray capacitance C.sub.d, shown by a broken line in FIG. 2, associated with the drain of the MOS FET S are charged by the operation voltage V.sub.V, from the power supply 5 so that the potential at each of the drain and source of the MOS FET S becomes V.sub.V.
When light falls on the photoelectric transducer D under such a condition, signal charge (electrons) is produced dependent on the quantity of the incident light, whereupon the potential of the source of the MOS FET S is lowered from V.sub.V. The potential of the source of the MOS FET S can be lowered down to ground and this drop of the source potential is expressed with .DELTA.V.sub.S hereon. Thereafter, the pluse .phi..sub.v of one .PHI..sub.v of the vertical scanning signals .PHI..sub.v1 to .PHI..sub.vm is supplied from the control terminal v to the gate of the MOS FET S to turn the latter on. Then, a signal based on the signal charge stored in the source of the MOS FET S is transferred to the drain of the MOS FET S, whereupon the potential at the drain of the MOS FET S is lowered by: ##EQU1## Since the capacitance C.sub.d is much larger than the junction capacitance C.sub.j, that is, C.sub.d &gt;&gt;C.sub.j, the reduction .DELTA.V.sub.d of the drain potential of the MOS FET S is so small that it can be regarded as substantially zero. Therefore, the potential of the drain of the MOS FET S is substantially V.sub.V while the signal is being transferred. Then, when the pulse .phi..sub.h of one .PHI..sub.h of the horizontal scanning signals .PHI..sub.h1 to .PHI..sub.hn is supplied from the control terminal h to the gate of the MOS FET T to turn the latter on, the signal transferred to the drain of the MOS FET S, that is, the source of the MOS FET T, is further transferred to the drain of the MOS FET T. A signal current due to the transferred signal then flows through the output resistor 4 so that an image pickup signal output is now available from the output terminal 6.
The source potential of the MOS FET S is now able to vary from V.sub.V to ground level. Assuming that the enhancement-mode MOS FET S has a threshold voltage V.sub.th (V.sub.th &gt;0), the level of the pulse .phi..sub.v of the vertical scanning signal .PHI..sub.v reguired to energize the MOS FET S to be conductive should reach at least V.sub.V +V.sub.th, and the low level portion of the vertical scanning signal .PHI..sub.v required to keep the MOS FET S to be nonconductive should be V.sub.th or smaller. Therefore, the vertical scanning signal .PHI..sub.v required to turn the MOS FET S on and off without fail should have the low level portion of a level of V.sub.th -V' and the pulse .PHI..sub.v of a level of V.sub.V +V.sub.th +V' (where V' is a marginal voltage).
Further, assuming that the enhancement-mode MOS FET T has also the threshold voltage V.sub.th, since the source potential of the MOS FET T varies from the level V.sub.V to the level V.sub.V -V.sub.d, the level of the pulse .phi..sub.h of the horizontal scanning signal .PHI..sub.h required to energize the MOS FET T to be conductive should reach at least V.sub.V +V.sub.th, and the low level portion of the horizontal scanning signal .PHI..sub.h required to keep the MOS FET T nonconductive should be V.sub.V -V.sub.d +V.sub.th or lower. Accordingly, the horizontal scanning signal .PHI..sub.h required to turn the MOS FET T on and off without fail should have the low level portion of a level of V.sub.V -V.sub.d +V.sub.th -V' and the pulse .phi..sub.h of the level of V.sub.V +V.sub.th +V'.
It follows from the foregoing that the level of the pulse .phi..sub.v of the vertical scanning signal .PHI..sub.v and the level of the pulse .phi..sub.h of the horizontal scanning signal .PHI..sub.h are required to be equal to each other, and therefore an attempt to equalize the low level portion of the horizontal scanning signal .PHI..sub.h to the low level portion of the vertical scanning signal .PHI..sub.v results in the fact that the pulse .PHI..sub.h of the horizontal scanning signal .PHI..sub.h and the pulse .phi..sub.v of the vertical scanning signal .PHI..sub.v have the same amplitude (V.sub.V +V.sub.th +V') -(V.sub.th -V')=V.sub.V +2V'. This imposes a greater burden on the horizontal scanning circuit, and the MOS FET T suffers from a disadvantage in respect to the potential durability thereof.
Further, if the low level portion of the horizontal scanning signal .PHI..sub.h is set to be the ground level, then the amplitude of the pulse .phi..sub.h would be increased further and this would result in the difficulty in designing the shift register constituting the horizontal scanning circuit. Besides, if the low level portion of the horizontal scanning signal .PHI..sub.h is set to be higher than the ground level, then the number of external terminals of the horizontal scanning circuit would be increased. This is disadvantageous in that the number of external pins of an integrated circuit unit, which are desired to be as few as possible, is increased in the case where the horizontal scanning circuit is formed into an integrated circuit.